Hair dye compositions for dyeing the hair are widely employed as, for example, “dyes for concealing gray (or, white) hair” or “dyes for imparting fashionable color.” Such a hair dye composition contains, as a matter of course, a coloring agent for dyeing the hair a desired color. Among such coloring agents, an “acid dye,” which exerts its hair dyeing effects to the maximum extent in an acidic region, is considered to be one of the most desirable coloring agents, since an acid dye exhibits excellent safety. Particularly, an azo acid dye is typically employed as a coloring agent for “semi-permanent hair dye compositions,” which are designed to impart semi-permanent dyeing effects to the hair. Therefore, there is a great need for an acid dye in hair dye compositions.
During use of an “acidic hair dye composition,” which contains an acid dye as a coloring agent, the pH of the composition must be regulated to fall within a strongly acidic region (i.e., pH of 1.5 to 4.5). In some cases, an acidic hair dye composition contains, in addition to an acid dye, a natural dyestuff in view of, for example, dyeability and color tone. When a natural dyestuff is incorporated into an acidic hair dye composition, desirably, the composition is used in an acidic region, in order to reduce, for example, stimulation to the hair. Therefore, a natural dyestuff which can exerts its dyeing effects in such an acidic region is desirably employed. Meanwhile, in general, a thickner is incorporated into such a strongly acidic hair dye composition, in order to prevent the composition from running through the hair upon use and from coming into contact with an object other than the hair.
However, considerable limitations are imposed on the thickner which can be employed in such a strongly acidic pH region, and even the thickner that is considered to be usable in a strongly acidic region is not necessarily satisfactory as a thickner to be incorporated into a hair dye composition. Conventionally, xanthan gum has been generally employed as a thickner in acidic hair dye compositions (Japanese Patent Publication (kokoku) No. 2-32253). Meanwhile, there has been proposed a hair dye composition containing, in addition to xanthan gum, bentonite and/or cross-linked sodium polyacrylate, which serves as an improving agent (Japanese Patent Application Laid-Open (kokai) No. 5-51309.
However, although a conventionally known hair dye composition containing xanthan gum is stable at a pH of 4.5 or higher, the composition undergoes considerable change in viscosity in a strongly acidic region (i.e., pH of 1.5 to 4.5), in which the composition exerts its dyeing effects; i.e., the composition involves problems in terms of stability. In addition, the hair dye composition exhibits poor fluidity, and when the composition is applied to the hair with the hand, in some cases, the composition forms a lump and runs off through the hair, or the composition is difficult to spread on the hair. Furthermore, the hair dye composition provides a sticky sensation upon use; i.e., the composition does not provide satisfactory sensation upon use.
Therefore, as described above, a hair dye composition containing bentonite and/or cross-linked sodium polyacrylate in combination with xanthan gum has been proposed for improving the aforementioned fluidity. However, the hair dye composition still poses a problem in terms of stability, and also involves other problems; for example, the composition cannot be easily rinsed off after use thereof.
Conventionally, there has been incorporated, into a product which is applied to the hair and then rinsed off, a rinse gel which is prepared from a combination of a long-chain alkyl alcohol and a cationic surfactant such as an alkyltrimethylammonium salt, in order to impart smoothness to the hair and reduce unruly dryness of the hair. However, such a cationic rinse gel cannot be incorporated into an acidic hair dye composition, since, when the dye contained in the composition, which is anionic, coexists with a cationic component, an ionic complex is formed in the composition, leading to generation of insoluble precipitates in the composition. Therefore, a conventional acidic hair dye composition involves problems in terms of sensation upon use, including any tactile frictional resistance between hair fibers during rinsing, and unruliness of the hair after the hair is dried.
In view of the foregoing, an object of the present invention is to provide a hair dye composition which is stable even in a strongly acidic region (pH of 1.5 to 4.5) in which an acid dye exerts its dyeing effects, which exhibits appropriate viscosity, which provides excellent sensation upon use, and which can be easily rinsed off after use thereof.